Interferometric synthetic aperture radar (“IFSAR”) is a radar system used to obtain target height information and form multidimensional maps of imaged areas. IFSAR utilizes at least two (2) SAR images of the same scene, formed at slightly different elevation angles relative to each other, to extract information about target heights. Such images can be coherently combined to ascertain the topography of the imaged area and produce three-dimensional maps of the imaged area.
Currently, IFSAR utilizes two (2) principal operational modes. The first mode is two-antenna, one-pass IFSAR in which a single aircraft with two (2) antennas, displaced in a direction normal to the flight path of the aircraft, flies by the scene of interest once while collecting data. The second mode is one-antenna, two-pass IFSAR in which a single aircraft with a single antenna flies by the scene of interest twice, along slightly offset flight paths, while collecting data.
Each of these IFSAR operation modes possesses strengths and weaknesses. The two-antenna, one-pass mode allows for precise knowledge of baseline length, but often limits that baseline length. Additionally, residual system noise manifests itself as uncertainty in the topographic height estimates with lesser baseline lengths resulting in increased height-noise and limiting the effective topographic resolution. Because the one-antenna, two-pass mode allows a greater separation of the antennas, it allows a larger baseline, and thereby has a diminished sensitivity to system noise. Although it produces less height-noise, it suffers from imprecise knowledge of the baseline length (antenna separation) between passes, and consequently does not provide accurate target height scaling. Conventional navigation instruments are inadequate in measuring aircraft flight paths with suitable precision. Although a two-antenna, two-pass IFSAR has the potential to provide both the scaling accuracy of the two-antenna, one-pass mode and the height-noise performance of the one-antenna, two-pass mode, it currently does not have precise knowledge of the baseline length between passes.
It is well-known by workers in the art that the uncertainty in the target height decreases as the antenna baselines increases. By decreasing the uncertainty in the target's height, the accuracy of the resulting digital elevation model improves. One alternative for increasing the antenna baseline length entails equipping a single aircraft with additional antennas with suitable offsets to generate greater baselines. This is expensive and complicates the engineering. Additionally, flexing of the aircraft structure during flight may also be severe enough to render unacceptable uncertainty in the larger baselines without somewhat expensive baseline measurement schemes or devices.
It is therefore desirable to provide a solution that avoids the weaknesses of the aforementioned conventional IFSAR modes.
Exemplary embodiments of the invention permit data driven alignment of multiple independent passes, thereby providing the scaling accuracy of the two-antenna, one-pass mode and the height-noise performance of the one-antenna, two-pass mode. The antenna baseline between multiple flight passes can be accurately estimated from the data itself, thereby reducing both height-noise and scaling error and allowing for more accurate information about target ground position locations and heights.